Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, are diagnostic of the petrophysical properties of the formation and the fluids therein. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a transmitter (such as a current electrode) is uses in conjunction with a diffuse return electrode (such as the tool body). A measured electric current flows in a circuit that connects a current source to the transmitter, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the first category.
With tools in the first category, it is desirable to know standoff distance between a measure electrode and a wall of a borehole. While high frequency electromagnetic signals are used to estimate the complex impedance between a measure electrode and a return electrode on a downhole tool, the resistivity of the formation may make obtaining accurate standoff estimates difficult. Additionally, electromagnetic induction resistivity well logging data may be limited by the accuracy of standoff information. The present disclosure addresses these difficulties.